As an arrangement similar to the electromagnetic clutch configured as above, PTL 1 discloses an arrangement wherein an input hub and an output hub are disposed in such a positional relationship that these hubs are rotatably mounted coaxially about a rotational axis and adjacent each other along the rotational axis. More particularly, in this disclosed arrangement, a wrap spring is mounted about the outer circumference of the input hub and the outer circumference of the output bub, and a control collar is disposed about the wrap spring, with one end of the wrap spring being supported to the input hub and the other end thereof being supported to the control collar.
In this PTL 1, when no magnetic force is applied to an armature provided in the control collar, there is provided a clutch-disengaged state in which the armature is maintained at a predetermined position by an urging force of a return spring, and the wrap spring is detached from the outer circumference of the input hub and the outer circumference of the output hub. Then, when a magnetic force is applied to the armature provided in the control collar, there is provided a state in which this magnetic force draws the armature toward the magnetic solenoid against the urging force of the return spring and a rotational force of the control collar is transmitted to the output hub. With this, there is reached a state wherein the coil diameter of the wrap spring is reduced, so that the wrap spring is brought into pressure-contact with the outer circumference of the input hub and the outer circumference of the output hub, thus transmitting the rotational drive force of the input hub to the output hub. In this way, a clutch-engaged state is realized.
Further, PTL 2 discloses an arrangement of an electromagnetic clutch as follows. In this, a wrap spring is mounted across/between an outer circumference of a drive pulley (an input pulley in this document) and an outer circumference of an output hub rotatable in union with a shaft (a pump shaft in this document). One end of this wrap spring is supported to an armature and the outer end of the spring is supported to the output hub. The drive pulley incorporates an electromagnetic solenoid for applying an attraction force to the armature.
In this PTL 2, when no magnetic force is applied from the electromagnetic solenoid to the armature, there is provided a clutch-disengaged state wherein the wrap spring is detached from the outer circumferences of the input pulley and of the output hub. When a magnetic force is applied from the electromagnetic solenoid to the armature, there is provided a clutch-engaged state wherein the wrap spring is brought into pressure-contact with the outer circumferences of the input pulley and the output hub, thus allowing transmission of the drive force of the drive pulley to the shaft.